Aspen facts

Aspen (Populus tremula) is characterised by its shimmering foliage in summer, the aspen tree is a beautiful but rare feature of the Caledonian Forest.

Global Distribution

European aspen is one of the most widely distributed trees in the world. Its natural range stretches from the Arctic Circle in Scandinavia to North Africa. It is found in Britain and across most of Europe and north Asia to China and Japan.

Distribution in Scotland

Although aspen occurs throughout Britain, it is most common in the north and west of Scotland. It is also known from Shetland and the Hebridean Islands. In the Highlands it will grow at elevations of up to 550 metres. In the Glen Affric area it is most often found on rocky slopes or cliffs with a southerly aspect. However, because it can tolerate a wide range of soil types and climatic conditions, it is likely that its present distribution is due to the effects of deforestation. These sites are places where it has been able to survive out of the reach of grazing animals.

In good situations, aspen can reach a height of twenty metres, but in most locations in the Highlands it will only grow to about ten metres tall. The bark is grey, or sometimes greenish-grey. (It can actually photosynthesize through its bark!) It is either smooth or in some cases pitted with diamond-shaped lenticels. On old, mature trees the bark is often covered with a dark-coloured lichen. This can give the trunk a black appearance. Aspen has a distinctive branching pattern, which is most visible in winter when the tree is leafless. In mature trees the topmost branches are often bent over horizontally.

Aspen leaves are round, measuring between 2.5 and 6 cm across. They have irregular blunt teeth on their margins. On young aspen suckers, or ramets, the leaves are usually elongated and triangular in shape. The leaf stalks, or petioles, of aspen are flattened and very flexible near the leaf blade. This gives rise to the characteristic fluttering of its leaves in the slightest breeze. When the leaves first open in spring, they are a coppery colour, before turning green. In autumn, the leaves turn a brilliant yellow, or in some cases red. Each separate aspen clone has its own individual colouration.

Like birch and rowan, aspen is a pioneer species. It is fast growing and regenerates profusely after disturbance. As in other pioneer species, an individual tree is short-lived, surviving for perhaps only 50-100 years. But the clone to which it belongs will live for much longer than this, if vegetative reproduction takes place. In North America scientists studied the longevity of the closely-related trembling aspen. They concluded that individual clones can survive for 14,000 years or more. This probably makes them the longest lived organisms on the planet!

Aspen is dioecious. This means that individual trees are either male or female. Most trees, such as Scots pine, have male and female flowers on the same tree. Aspen flowers in March and April, before the leaves appear, with both the male and female trees producing catkins. Pollinated female catkins ripen in early summer and release tiny seeds which are tufted with hairs. Each seed weighs about one ten-thousandth of a gram!

However, flowering and seed production by aspen rarely occur in Scotland. The reasons for this are not fully understood, but it restricts the scope for pollination between male and female trees. Our oceanic climate may be the cause, but it is also possible that aspen clones become less fertile as they age, and many stands could be very old. As woodland has become more fragmented it has less chance of reproducing even when it does flower. This leads to a lack of young trees.

However, even in other parts of its range where it is more abundant, aspen’s main method of reproduction is vegetative. New suckers, or ramets, grow off the roots of mature trees. The numbers of new shoots produced in this way can be very prolific, especially after a major disturbance such as fire.

Aspen has an extensive root system, and ramets have been recorded growing up to 40 metres from a parent tree. Aspen ramets can grow very quickly because they get nutrients through the parent tree’s root system. As the ramets grow, they remain joined through their roots. All the interconnected trees are called a clone. They are all the same individual organism and thus are all single-sexed. All the trees in a particular clone will come into leaf at the same time in the spring. You can sometimes identify a clone by the specific colour its leaves change to in the autumn.

Aspen roots also have the unusual ability of staying alive underground for many years after the death of the parent tree. This leads to the appearance of ramets in areas where there are no mature trees. There is a good example of this at a site on the north shore of Loch Beinn a’Mheadhion in Glen Affric.

Aspen is an important tree for biodiversity. There are a lot of organisms that use aspen and some of them depend on it completely. Young aspen shoots are a food source for caterpillars of the rare dark-bordered beauty moth. There are also a range of deadwood-dependent insects that rely on old decaying aspen. These include the endangered aspen hoverfly. This suite of insects needs aspen stands of over 4.5 hectares to remain viable.

Over 60 species of insect have been recorded as feeding on aspen foliage in Scotland. These include two gall midges which are restricted to this tree. Other invertebrates on aspen include a weevil called the aspen leaf roller and a mite that causes cauliflower galls.

The blunt-leaved bristle moss is only found on aspen and this tree is also valuable for many different lichens. The bark has a varied texture and low acidity and so it can host lichens that don’t grow on many other trees. A tiny and very rare pinhead lichen only seems to grow on aspen twigs.

Many species of fungus associate with aspen, and aspen bracket fungus only grows on this tree. While it causes decay in the trunk, it helps provide habitat for bats, woodpeckers and other wildlife. In fact studies from Scandinavia have shown that hollow aspens provide some of the best roosting sites for bats.

Aspen leaves are readily eaten by red deer. Overgrazing in this way is the main factor preventing natural regeneration in existing stands of aspen. It is also an important tree for the European beaver which feeds extensively on aspen when it can. This could pose something of a dilemma. If we want to see the expansion of this important rodent, won’t other species lose out? The strategy we have taken since the 1990s is to substantially increase the area of aspen so there is plenty to go around as beaver populations expand. This tree was once much more abundant in Scotland and we hope that one day it will be again.

 

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How to identify an aspen

 
Doug Gilbert explains how to identify this rare tree. Its shimmering leaves flutter like tiny flags in the wind, turning beautiful shades of yellow and red in the autumn.

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